The ability to detect biomolecules in minute quantities is of paramount importance both in biological research and in clinics. Quantitative detection of biomolecules in a specific sample can reveal significant information ranging from cell decision making processes to state of a specific disease. For example, the disease of cancer, that is responsible for half-million deaths in the U.S. every year, can potentially be cured if diagnosed early. One way to perform the diagnosis is to detect tumor markers in body fluids. For a successful early diagnosis, the detection platform must certainly be sensitive and reveal the presence of small amounts of target molecules. The detection platform must also be miniaturizable, quick, relatively simple to operate and inexpensive to enable 1) high- throughput detection and 2) sufficient number of detection platforms in a setting (clinics, research laboratory). Most detection platforms that currently exist require the use of labeling molecules such as fluorescent and radio labels. This increases time, cost and required expertise for operation and renders these label-based platforms not necessarily suitable for miniaturization and high-throughput detection. This proposal covers the development of a biomolecular detection platform that is fluorescence and radio labelfree, simple to fabricate and use, robust and at the same time more sensitive than many existing detection platforms. The proposed system is based on immunomagnetic separation and diffractometry whose combination has not yet been explored. Functionalized magnetic beads are used to capture target molecules from solutions. After this step, the beads (that contain the captured molecules on their surfaces) are simply exposed to a solid surface that is pre-functionalized with secondary receptor molecules. The functionalization is done in an alternating stripe pattern, so that the binding of the beads forms a solid diffraction grating. The illumination of the bound beads forms diffraction modes whose intensities reveal the presence of the beads, and hence the target molecules captured. The detection system does not require any additional amplification steps or fluorescence or radio labels for signaling. The study has 2 PIs and 3 specific aims. Aim 1 is the development of a mathematical model to predict the optical behavior of a bead-based diffraction system. The model will be based on a Monte Carlo simulation and will predict the optimal system parameters (eg. bead size, grating periodicity, surface material, refractive indices) for maximum sensitivity. Aim 2 is the development of the actual setup which will start simultaneously with Aim 1 but later on adapt its results to the experimental setup. Aim 2 will also optimize the system experimentally in terms of receptor and ligand concentrations and background dilutions levels, and explore the limits of detection. Aim 3 is the application of the system to detection of a known tumor marker: vascular endothelial growth factor (VEGF) in complex mixtures such as sera and cell lysate. This aim will explore the "clinical usefulness" of the system with the goal of achieving 500 fM detection sensitivity, a concentration change that distinguishes cancer patients from healthy people.